The way forward of feedback as agreed in 3GPP RAN #60 decides that a precoder for a subband is composed of two matrices, one of which targets at wideband and/or long-term channel properties, and the other matrix targets at frequency-selective and/or short-term channel properties. Codebooks may or may not change/vary over time and/or different subbands.
Generally, in a system, one of a long-term wideband beamforming vector and a long-term wideband transmit spatial correlation matrix is fed back as a long-term wideband channel property feedback from a mobile station to a base station based on a downlink status.
As two different forms of long-term wideband information available to a transmitter, the long-term wideband beamforming vector and the long-term wideband transmit spatial correlation matrix have been widely discussed in the standardization process of LTE-Advanced, wherein the long-term wideband beamforming matrix is for forming a beam pattern (i.e., a precoding matrix) for a receiver in a single-user multi-input multi-output/multi-input single-output wireless communications system, and the long-term wideband transmit spatial correlation matrix can be directly used to generate a precoding matrix, for example, a signal-to-leakage-plus-noise ratio (SLNR) used in the multi-user multi-input multi-output wireless communications system, or a transformed codebook which reduces the quantization error, etc.
In actuality, because improved channel state information (CSI) can not only benefit a single-user multi-input multi-output/multi-input single-output wireless system but also benefit the multi-user multi-input multi-output/multi-input single-output wireless communications system more significantly, it is commonly understood that quantization error should be reduced to the least. Current study shows that the transmit spatial correlation can help form a transformed codebook derived from a base codebook to better suit the characteristic of a user channel and further improve the system performance especially for the multi-user multi-input multi-output/multi-input single-output system. Thus, for downlink, a base station such as eNodeB must know the transmit spatial correlation information for each user equipment (UE). The transmit spatial correlation information, for example, the aforementioned long-term wideband transmit spatial correlation matrix, may be measured by UE and fed back to eNodeB. However, not all systems feed back the long-term wideband transmit spatial correlation matrix to the eNodeB. As above mentioned, UEs in some systems may merely feed back the long-term wideband beamforming matrix as the long-term wideband information to the eNodeB. Thus, in this case, the eNodeB cannot feed back the long-term wideband transmit spatial correlation matrix as obtained from the UE. Generally, what is fed back by the UE is the index corresponding to the long-term wideband beamforming matrix or long-term wideband transmit spatial correlation matrix as selected from the codebook.
Besides, those skilled in the art would appreciate that in some standards (for example, LTE-Advanced) only one form of long-term information can be prescribed, or in a particular application scenario, only one form of long-term and/or wideband information is allowed to use, i.e., long-term wideband beamforming matrix or long-term wideband spatial correlation matrix.
Therefore, it is desirable for a method capable of transforming one form of long-term and/or wideband information into another form of long-term and/or wideband information.